There has been a growing interest in the application of microfluidic systems to a variety of technical areas, including such diverse fields as biochemical analysis, medical diagnostics, chemical synthesis, and environmental monitoring. For example, use of microfluidic systems for acquiring chemical and biological information presents certain advantages. In particular, microfluidic systems permit complicated processes to be carried out using very small volumes of fluid. In addition to minimizing sample volume, microfluidic systems increase the response time of reactions and reduce reagent consumption. Furthermore, when conducted in microfluidic volumes, a large number of complicated biochemical reactions and/or processes may be carried out in a small area, such as in a single integrated device. Examples of desirable applications for microfluidic technology include analytical chemistry; chemical and biological synthesis, DNA amplification; and screening of chemical and biological agents for activity, among others.
Traditional methods for constructing microfluidic devices have used surface micromachining techniques borrowed from the silicon fabrication industry. According to these techniques, microfluidic devices have been constructed in a planar fashion, typically covered with a glass or other cover material to enclose fluid channels. Representative devices are described, for example, in some early work by Manz, et al. (Trends in Anal. Chem. (1990) 10(5): 144-149; Advances in Chromatography (1993) 33: 1-66). These publications describe microfluidic devices constructed using photolithography to pattern channels on silicon or glass substrates, followed by application of surface etching techniques to remove material from a substrate to form channels. Thereafter, a cover plate is typically to the top of an etched substrate to enclose the channels and contain a flowing fluid.
More recently, a number of methods have been developed that allow microfluidic devices to be constructed from plastic, silicone or other polymeric materials. Fabrication methods include micromolding of plastics or silicone using surface-etched silicon as the mold material (see, e.g., Duffy et al., Anal. Chem. (1998) 70: 4974-4984; McCormick et al., Anal. Chem. (1997) 69: 2626-2630); injection-molding; and micromolding using a LIGA technique (see, e.g., Schomburg et al., Journal of Micromechanical Microengineering (1994) 4: 186-191), as developed at the Karolsruhe Nuclear Research Center in Germany and commercialized by MicroParts (Dortmund, Germany). LIGA and hot-embossing techniques have also been demonstrated by Jenoptik (Jena, Germany). Imprinting methods in polymethylmethacrylate (PMMA) have also been described (see, e.g., Martynova et al., Anal. Chem. (1997) 69: 4783-4789). These various techniques are typically used to fashion planar (i.e., two dimensional, or 2-D) structures that require some sort of cover to enclose microfluidic channels. Additionally, these techniques do not lend themselves to rapid prototyping and manufacturing flexibility. Moreover, the tool-up costs for such techniques are often quite high and can be cost-prohibitive
A more recent method for constructing microfluidic devices uses a KrF laser to perform bulk laser ablation in fluorocarbons that have been compounded with carbon black to cause the fluorocarbon to be absorptive of the KrF laser (see, e.g., McNeely et al., “Hydrophobic Microfluidics,” SPIE Microfluidic Devices & Systems IV, Vol. 3877 (1999)). This method is reported to reduce prototyping time; however, the addition of carbon black renders the material optically impure and presents potential chemical compatibility issues. Additionally, the reference is directed only to planar structures.
When working with fluids in conventional macroscopic volumes, splitting a fluid stream into two or more portions or substreams is a relatively straightforward task. Generally, bulk fluid interactions have a greater effect on fluid flow behavior than interactions between the fluid and confining surfaces. It is relatively simple to construct fluid splitting systems that will fill in a predictable manner.
In microfluidic systems, however, achieving consistent and predictable splitting is not a trivial matter. Microfluidic systems are characterized by extremely high surface-to-volume ratios, causing surface interactions to have a much more significant effect on fluid movement. In simple terms, if a fluid stream is provided to a junction or manifold region having multiple outlet channels (such as the manifolded prior art splitting design illustrated in FIG. 1), it is difficult to predict which one or more of the outlet channels will be filled. For example, if an advancing fluid front in a microfluidic system encounters a forked region and flow is established in one branch of the fork, there is little impetus for flow to be initiated in the other branch.
It is desirable to produce relatively compact microfluidic systems to promote easy interface with standard laboratory instruments including detection instruments such as plate readers and dispensing equipment including automated pipettors. It is also desirable to provide microfluidic devices capable of multiple simultaneous operations with a minimal number of fluidic interfaces. As a result, it would be desirable to provide splitting utility on a microfluidic device in as compact an area as possible.
A method for controlling fluid splitting in microfluidic channels has been proposed in U.S. Pat. No. 6,296,020 (“the '020 reference”), issued on Oct. 2, 2001 to assignee BioMicro Systems, Inc. A splitting channel network including a series of “daughter” channels is defined in a single layer using micromachining techniques, with short channel narrowings or restrictions disposed at the far end of each channel to provide a pressure barrier. A splitting channel network according to the design of the '020 reference is illustrated in FIG. 2. Each generation of channel restrictions needs to provide a greater pressure barrier than the previous generation in order to promote predictable splitting. Devices according to the '020 reference, however, suffer from defects that limit their utility. To begin with, it is difficult and time-consuming to fabricate microfluidic devices with even the simplest micromachining techniques. Predictable splitting systems confined to a single device layer inherently consume a relatively large footprint on a microfluidic device, particularly when it is desirable to split to a large number of outlet channels. Additionally, the progressively increasing pressure barriers proposed in the '020 reference impose a practical limit to the number of splits that can be achieved. Notably, the '020 reference discloses no more than 4-way splitting.
In certain applications, precise and uniform splitting would be highly desirable. For example, highly parallel chemical and biological separation techniques such as liquid chromatography have been proposed, to achieve multiple separations simultaneously. Chromatography is a physical method of separation wherein components partition between two phases: a stationary phase and a mobile phase. Sample components are carried by a mobile phase through a bed of stationary phase.
In liquid chromatography applications, it is often desirable to alter the makeup of the mobile phase during a particular separation, such as by mixing two or more mobile phase components in different proportions. If multiple separation columns are provided in a single integrated (highly parallel) device and the makeup of the mobile phase is subject to change over time, then at a common linear distance from the mobile phase inlet it is desirable for mobile phase to have a substantially identical composition from one column to the next.
In light of the foregoing, there exists a need for a microfluidic splitter that is compact, easy to fabricate, and is scalable to provide uniform splitting to a large number of outlet channels.